The present invention relates to an image forming material comprising a so-called positive working light sensitive layer to be solubilized by an active light radiation and an image forming method employing the same, and particularly to an image forming material, for example, a positive working presensitized planographic printing plate, capable of forming an image by an infrared light such as a semiconductive laser and an image forming method employing the same.
As a positive working light sensitive layer to be solubilized by an active light radiation a light sensitive layer containing an acid generating compound and an acid decomposable compound is known. A light sensitive composition containing a compound having an orthocarboxylic acid or carboxylic acid amide acetal group is disclosed in U.S. Pat. No. 3,779,779, a light sensitive composition containing a compound having an acetal or a ketal in the main chain is disclosed in Japanese Patent O.P.I. Publication No. 53-133429/1978, and a light sensitive composition containing a compound having a silylether group is disclosed in Japanese Patent O.P.I. Publication No. 65-37549/1985. These compounds have sensitivity in the ultraviolet range and those capable of being solubilized by ultraviolet radiation, and can not give an image by irradiation of an infrared light such as a cheap and compact semiconductor laser.
As a technique giving an image by irradiation of an infrared light such as a semiconductor laser, a light sensitive layer containing an acid generating compound, a resol resin, a novolak resin and an infrared absorber is disclosed in compound capable of generating an acid on radiation of an active light is disclosed in U.S. Pat. No. 5,340,699, which gives a negative image by imagewise exposing, heating and developing and a positive image by imagewise exposing and then developing without heating. However, in this technique, the negative image forming method requires additional heating process incovenient to users and imposes much electric power consumption, and on the other hand, the positive image forming method has the disadvantages that stains occur in non-image portions due to poor development, a part of the image portions is dissolved in a developer. Further, storage stability of an image forming material comprising this light sensitive layer is not satisfactory.
An object of the invention is to provide a positive working image forming material capable of giving an image by irradiation of infrared rays and an image forming method employing the same.
Another object of the invention is to provide to provide a positive working image forming material containing an acid generating compound and an acid decomposable compound which is capable of giving an image by irradiation of infrared rays and an image forming method employing the same.
Still another object of the invention is to provide a positive working image forming material having high sensitivity to infrared rays and excellent developability and storage stability and an image forming method employing the same.
The above object of the invention can be attained by the following:
(1) An image forming material comprising a support and provided thereon, a light sensitive layer containing a compound capable of generating an acid on irradiation of an active light, a compound having at least one chemical bond capable of being decomposed by an acid and an infrared absorber,
(2) The image forming material of (1) above, wherein the compound capable of generating an acid is an organic halogen containing compound,
(3) The image forming material of (1) above, wherein the compound having at least one chemical bond capable of being decomposed by an acid is a compound having a xe2x80x94(CH2CH2O)nxe2x80x94 in which n is an integer of 2 to 5,
(4) The image forming material of (1) above, wherein the compound having at least one chemical bond capable of being decomposed by an acid is a compound represented by the following formula (1): 
xe2x80x83Wherein m and n independently represent an integer of 1 to 5; R, Rxe2x80x2 and Rxe2x80x3 independently represent a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, an alkoxy group, a hydroxy group, a sulfo group or its salt group or a carboxyl group or its salt group; and p, q and r independently represent an integer of 1 to 3,
(5) The image forming material of (1) above, wherein the infrared absorber is a cyanine dye represented by the following formula (2): 
wherein Z1 represents a sulfur, selenium, oxygen atom or C (R3) (R4) in which R3 and R4 independently represent a hydrogen atom, a halogen atom or an alkyl group having 1 to 3 carbon atoms; Z2 represents a sulfur, selenium, oxygen atom or C (R5) (R6) in which R5 and R6 independently represent a hydrogen atom, a halogen atom or an alkyl group having 1 to 3 carbon atoms; X1 and X2 independently represent an atomic group necessary to form a benzene ring or a condensed ring; R1 and R2 independently represent a substituent, provided that one of R1 and R2 represents an anion; and L represents a conjugated bond chain having 5 to 13 carbon atoms.
(6) The image forming material of (1) through (5) above, wherein the light sensitive layer further comprises a novolak resin and a nonionic surfactant,
(7) The image forming material of (1) through (5) above, wherein the light sensitive layer further comprises a novolak resin and an acryl resin, or
(8) An image forming method comprising the steps of imagewise exposing the image forming material of (1) through (7) above to a visible or infrared light having a wavelength of 700 nm or more; and removing the exposed portions with an alkaline developer.
The present invention will be detailed below.
The compound (hereinafter referred to as the acid generating compound of the invention) capable of generating an acid on irradiation of an active light used in the light sensitive layer of the invention includes various conventional compounds and mixtures. For example, a salt of diazonium, phosphonium, sulfonium or iodonium ion with BF4xe2x88x92, PF6xe2x88x92, SbF6xe2x88x92 SiF62xe2x88x92 or ClO4xe2x88x92, an organic halogen containing compound, o-quinonediazide sulfonylchloride or an organic metal is a compound capable of generating or releasing an acid on irradiation of an active light and can be used as the acid generating compound of the invention. The organic halogen containing compound known as an photoinitiator capable of forming a free radical forms a hydrogen halide and can be used as the acid generating compound of the invention.
The examples of the organic halogen containing compound capable of forming a hydrogen halide include those disclosed in U.S. Pat. Nos. 3,515,552, 3,536,489 and 3,779,778 and West German Patent No. 2,243,621, and compounds generating an acid by photodegradation disclosed in West German Patent No. 2,610,842.
Further, 2-halomethyl-1,3,4-oxadiazole compound, for example, 2-trichloromethyl-5-[xcex2(2-benzofuryl)vinyl]-1,3,4-oxadiazole can be used. The examples of the acid generating compounds of the invention include those disclosed in Japanese Patent O.P.I. Publication No. 56-17345/1981.
The o-naphthoquinone diazide-4-sulfonylhalogenide disclosed in Japanese Patent O.P.I. Publication No. 50-36209/1975.
In the invention the organic halogen containing compound is preferable for an acid generating compound in view of sensitivity to infrared rays or storage stability of an image foming material using it. In the organic halogen containing compound, triazines or oxadiazoles having a halogen-substituted alkyl group are preferable, and triazines having a halogen-substituted alkyl group are especially preferable.
The examples of the oxadiazoles having a halogen-substituted alkyl group include a 2-halomethyl-1,3,4-oxadiazole compound disclosed in Japanese Patent O.P.I. Publication Nos. 54-74728/1979, 55-24113/1980, 55-77742/1980, 60-3626/1985 and 60-138539/1985. Of these compounds, 2-trichloromethyl-5-[xcex2(2-benzofuryl)vinyl]-1,3,4-oxadiazole and the following acid generating compounds (1) and (2) are preferable. 
Of the triazines having a halogen-substituted alkyl group is preferable a compound represented by the following formula (3): 
wherein R represents an alkyl group, a halogen-substituted alkyl or aryl (for example, phenyl or naphthyl) group or its substituent; X represents a halogen atom.
The examples of s-triazines represented by formula (3) are listed. 
The content of the acid generating compound is preferably about 0.1 to 20% by weight, and more preferably 0.2 to 10% by weight based on the total weight of light sensitive layer, although the content broadly varies depending on its chemical properties or a composition or physical property of the light sensitive layer.
The compound (hereinafter referred to as the acid decomposable compound of the invention) comprising at least noe chemical bond capable of being decomposed by an acid used in the light sensitive layer of the invention includes a compound having a Cxe2x80x94Oxe2x80x94C bond disclosed in Japanese Patent O.P.I. Publication Nos. 48-89003/1973, 51-120714/1976, 53-133429/1978, 55-12995/1980, 55-126236/1980 and 56-17345/1981, a compound having a Sixe2x80x94Oxe2x80x94C bond disclosed in Japanese Patent O.P.I. Publication Nos. 60-37549/1985 and 60-121446/1985, another acid decomposable compound disclosed in Japanese Patent O.P.I. Publication Nos. 60-3625/1985 and 60-10247/1985, a compound having a Sixe2x80x94N bond disclosed in Japanese Patent O.P.I. Publication No. 62-222246/1987, a carbonic acid ester disclosed in Japanese Patent O.P.I. Publication No. 62-251743/1987, an orthocarbonic acid ester disclosed in Japanese Patent O.P.I. Publication No. 62-209451/1987, an orthotitanic acid ester disclosed in Japanese Patent O.P.I. Publication No. 62-280841/1987, an orthosilicic acid ester disclosed in Japanese Patent O.P.I. Publication No. 62-280842/1987, an acetal or ketal disclosed in Japanese Patent O.P.I. Publication No. 63-10153/1988 and a compound having a Cxe2x80x94S bond disclosed in Japanese Patent O.P.I. Publication No. 62-244038/1987.
Of these compounds, the compound having a Cxe2x80x94Oxe2x80x94C bond, the compound having a Sixe2x80x94Oxe2x80x94C bond, the orthocarbonic acid ester, the acetals or ketals or the silylethers disclosed in Japanese Patent O.P.I. Publication Nos. 53-133429/1978, 56-17345/1981, 60-121446/1985, 60-37549/1985, 62-209451/1987 and 63-10153/1988 are preferable.
Of these compounds is especially preferable a polymer disclosed in Japanese Patent O.P.I. Publication No. 53-133429/1978 which has a repeated acetal or ketal group in the main chain and increasing solubility in a developer by action of an acid or a compound capable of being decomposed by an acid disclosed in Japanese Patent O.P.I. Publication No. 63-10153/1988, which has the following structure: 
Wherein X represents a hydrogen atom or 
Y represents 
provided that X and Y may be the same or different.
The examples of the acid decomposable compound of the invention include compounds disclosed in the above described patent specification and their synthetic method is described in the above described patent specification.
The content of the acid decomposable compound of the invention is preferably 5 to 70% by weight, and more preferably 10 to 50% by weight based on the total weight of light sensitive layer. The acid decomposable compound of the invention can be used singly or in combination.
As the acid decomposable compound of the invention, a compound having a xe2x80x94(CH2CH2O)nxe2x80x94 group in which n is an integer of 2 to 5, in view of sensitivity and developability. Of the compound having a xe2x80x94(CH2CH2O)nxe2x80x94 group, n is especially preferably 3 or 4.
The typical example of such a compound includes a condensation product of dimethoxycyclohexane, benzaldehyde or their derivative with diethylene glycol, triethylene glycol, tetraethylene glycol or pentaethylene glycol.
In the invention, the compound represented by formula (1) above is preferable as an acid decomposable compound in view of sensitivity and developability. In the compound represented by formula (1) above, m and n each especially preferably are 1 or 2.
As the infrared absorber used in the light sensitive layer of the image forming material in the invention, an infrared absorbing dye having an absorption in the wavelength range of 700 nm or more. The especially preferable infrared absorber has an absorption maximum in the wavelength range of 700 nm to 850 nm and having a molar extinction coefficient, xcex5 of 105 or more.
The above infrared absorber includes cyanine dyes, squarium dyes, croconium dyes, azulenium dyes, phthalocyanine dyes, naphthalocyanine dyes, polymethine dyes, naphthoquinone dyes, thiopyrilium dyes, dithiol metal complex dyes, anthraquinone dyes, indoaniline metal complex dyes and intermolecular charge transfer complex dyes.
The above described infrared absorber includes compounds disclosed in Japanese Patent O.P.I. Publication Nos. 63-139191/1988, 64-33547/1989, 1-160683/1989, 1-280750/1989, 1-293342/1989, 2-2074/1990, 3-26593/1991, 3-30991/1991, 3-34891/1991, 3-36093/1991, 3-36094/1991, 3-36095/1991, 3-42281/1991 and 3-103476/1991.
In the invention the cyanine dye represented by formula (2) above is especially preferable as an infrared absorber. The cyanine dye represented by formula (2) includes those which comprise formula (2) as a cation and a counter anion. The counter anion includes Clxe2x88x92, Brxe2x88x92, Cl4Oxe2x88x92, BF4xe2x88x92 or an alkyl boronium ion such as t-butyltriphenylboronium ion.
In formula (2), the carbon number of the conjugated bond chain represented by L is preferably selected according to the wavelength emitted from an infrared laser used as a light source for imagewise exposure. For example, when a YAG laser having a wavelength of 1060 nm is used, the number is preferably 9 to 13. The conjugated bond chain may have a substituent, and may form a ring with an atomic group necessary to form a ring. In formula (2), the ring represented by X1 or X2 may have a substituent which preferably is a halogen atom, an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, xe2x80x94SO3M or xe2x80x94COOM in which M represents a hydrogen atom or an alkali metal. One of R1 and R2 preferably represents an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, xe2x80x94[(CH2)nO]kxe2x80x94(CH2)mOR in which n and m each are an integer of 1 to 3, k is 0 or 1 and R represents an alkyl group having 1 to 5 carbon atoms, xe2x80x94R1SO3M or xe2x80x94R1COOM in which R1 represents an alkylene group having 1 to 5 carbon atoms and M represents a hydrogen atom or an alkali metal, and the other preferably represents xe2x80x94R1SO3xe2x88x92 or xe2x80x94R1COOxe2x80x94 in which R1 represents an alkylene group having 1 to 5 carbon atoms, which forms an intramolecular salt in view of sensitivity or developability.
When a semiconductor laser is used as a light source for imagewise exposure, the cyanine dye represented by formula (2) having an absorption peak in the wavelength range of 750 to 900 nm and having a molar extinction coefficient, xcex5 exceeding 1xc3x97105. When a YAG laser is used as a light source for imagewise exposure, the cyanine dye represented by formula (2) having an absorption peak in the wavelength range of 900 to 1200 nm and having a molar extinction coefficient, xcex5 exceeding 1xc3x97105. The infrared absorber of the light sensitive layer is preferably 0.5 to 5% by weight.
The examples of the infrared absorber preferably used in the invention are listed below, but are not limited thereto. 
These dyes can be obtained by a conventional synthetic method, and the following commercially available dyes can be used:
IR750 (antraquinone type); IR002 and IR003 (aluminium type), IR820 (polymethine type); IRG022 and IRG033 (diimmonium type); CY-2, CY-4, CY-9 and CY-20, each produced by Nihon Kayaku Co., Ltd;
KIR103 and SIR103 (phthalocyanine type); KIR101 and SIR114 (antraquinone type); PA1001, PA1005, PA1006 and SIR128, (metal complex type), each produced by Mitsui Toatsu Co., Ltd;
FATOGEN BLUE 8120 produced by Dainihon Ink Kagaku Co., Ltd.; and
MIR-101,1011, and 1021 each produced by Midori Kagaku Co., Ltd.
Other infrared dyes are sold by Nihon Kankoshikiso Co., Ltd., Sumitomo Kagaku Co., Ltd. or Fuji Film Co., Ltd.
A binder can be used in the light sensitive layer of the image forming material of the invention. A polymer binder can be used as the binder. The binder includes a novolak resin, a polyhydroxystyrene, a polymer having a structural unit represented by formula (4) and another conventional acryl resin. The binder content of the light sensitive material is preferably 5 to 90 weight %.
The novolak resin includes a phenol-formaldehyde resin, a cresol.formaldehyde resin, a phenol.cresol.formaldehyde resin disclosed in Japanese Patent O.P.I. Publication No. 55-57841/1980 and a polycondensation resin of a p-substituted phenol or phenol and cresol with formaldehyde.
The polyhydroxystyrene includes a homopolymer or copolymer of hydroxystyrene disclosed in Japanese Patent Publication No. 52-41050/1977.
The polymer having a structural unit represented by formula (4) includes a homopolymer having only the structural unit represented by formula (4) and a copolymer having the structural unit represented by formula (4) and another acryl monomer unit. 
wherein R1 and R2 independently represent a hydrogen atom, an alkyl group such as methyl or ethyl or a carboxyl group, and preferably a hydrogen atom; R3 represents a hydrogen atom, a halogen atom such as chlorine or bromine or an alkyl group such as methyl or ethyl, and preferably a hydrogen atom or methyl; R4 represents a hydrogen atom, an alkyl group such as methyl, an aryl group such as a phenyl group or a naphthyl group; Y represents a substituted or unsubstituted phenylene or naphthylene group, the substituent including an alkyl group such as methyl or ethyl, a halogen atom such as chlorine or bromine; a carboxyl group, an alkoxy group such as methoxy or ethoxy, a hydroxy group, a sulfonic acid group, a cyano group, a nitro group or an acyl group, and preferably a unsubstituted phenylene or naphthylene group or a methyl substituted phenylene or naphthylene group; X represents an organic divalent group combining a nitrogen atom with a carbon atom in the aromatic ring; and n is an integer of 0 to 5, and preferably 0.
The polymer having a structural unit represented by formula (4) includes a polymer represented by the following formulas (a) through (h):
In formulas (a) through (h), R1 through R5 independently represent a hydrogen atom, an alkyl group or a halogen atom; X represents an alkyl group or a halogen atom; and m, n, l, k and s independently represent mol %.
The novolak resin, poly hydroxystyrene, polymer having a structural unit represented by formula (4) and another conventional acryl resin can be used in combination.
The light sensitive layer preferably contains a nonoionic surfactant. The nonoionic surfactant includes polyoxyethylene alkylether, polyoxyethylene alkylarylether, polyoxyethylene derivatives, oxyethylene.oxypropylene blockpolymer, sorbitan aliphatic acid ester, polyoxyethylene sorbitan aliphatic acid ester, polyoxyethylene sorbitol aliphatic acid ester, glycerin aliphatic acid ester, polyoxyethylene aliphatic acid ester, polyoxyethylene alkylamine and polyoxyethylene alkanolamide.
The light sensitive layer in the invention may contain a lipophilic resin to increase lipophilicity of the layer. The lipophilic resin includes a polycondensate of phenols with an alkyl group having 3 to 15 carbon atoms with aldehydes, for example, a t-butylphenol.formaldehyde resin disclosed in Japanese Patent O.P.I. Publication No. 50-125806/1975. The light sensitive layer in the image forming material of the invention may optionally contain dyes other than the dyes deescribed above, pigment or a sensitizer.
The light sensitive layer in the invention is provided on a support by dissolving the components described above in the following solvent, coating the solution on the support and drying, whereby the image forming material of the invention is obtained. The solvent includes propylene glycol monomethylether, propylene glycol monoethylether, methylcellosolve, methylcellosolve acetate, ethylcellosolve, ethylcellosolve acetate, dimethylformamide, dimethylsulfoxide, dioxane, acetone, cyclohexanone, trichloroethylene and methylethyl ketone. These solvents can be used singly or in combination.
The coating method includes conventional coating methods such as a whirler coating method, a wire-bar coating method, a dip coating method, an air-knife coating method, a blade coating method and a curtain coating method. The coating amount of the light sensitive layer in a presensitized planographic printing plate is preferably 0.5 to 5.0 g/m2, although it varies depending on the usage.
The support, on which the light sensitive layer is provides, includes a metal plate such as aluminium, zinc, steel or copper, a metal plate, paper sheet, plastic film or glass plate which is plated or vacuum evaporated with chromium, zinc, copper, nickel, aluminium or iron, a paper sheet coated with a resin, a paper sheet laminated with a metal foil such as aluminium and a plastic film subjected to hydrophilic treatment. Of these, an aluminium plate is preferable. When the invention is applied to a presensitized planographic printing plate, the support is preferably an aluminium plate which is subjected to a surface treatment such as graining treatment, anodizing treatment or sealing treatment. The surface treatment is carried out by a conventional method.
The graining treatment includes a mechanically graining method and an electrolytically etching method. The mechanically graining method includes a ball graining method, a brush graining method, a liquid horning graining method and a buff graining method. The above methods can be used singly or in combination according to an aluminium material composition. The electrolytically etching is carried out in a bath containing one or more of phosphoric acid, sulfuric acid, hydrochloric acid and nitric acid. After graining, the surface of the support is optionally subjected to desmut treatment using an alkaline or acid solution to neutralize and washed with water.
The anodizing is carried out by electrolyzing the surface of the aluminium support using the aluminium plate as an anode in a solution containing one or more of sulfuric acid, chromic acid, oxalic acid, phosphoric acid and malonic acid. The thickness of the anodizing film formed is suitably 1 to 50 mg/dm2, preferably 10 to 40 mg/dm2, and more preferably 25 to 40 mg/dm2. The thickness of the anodizing film is obtained by immersing the anodized aluminium in a solution containing phosphoric acid and chromic acid (water is added to 35 ml of 85% phosphoric acid and 20 g of chromium (IV) oxide to make a 1 liter solution) to dissolve the anodized film and measuring the aluminium weight before and after the immersing.
The sealing is carried out by treating the aluminium support with a boiling water, steam, a sodium silicate solution or a dichromic acid solution
The image forming material of the invention is imagewise exposed to a light source having a wavelength of 700 nm or more. The light source includes a semiconductor laser, a Hexe2x80x94Ne laser, a YAG laser, and a carbon dioxide laser. The output power is suitably 50 mW or more, and preferably 100 mW or more.
The image forming material of the invention is developed with an aqueous alkaline developer. The aqueous alkaline developer includes an aqueous solution containing an alkali metal salt such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium metasilicate, potassium metasilicate or di or trisodium phosphate. The metal salt concentration of the developer is preferably 0.05 to 20% by weight, and more preferably 0.1 to 10% by weight. The developer optionally contains an anionic surfactant, an amphoteric surfactant or an organic solvent such as alcohol. The organic solvent includes propylene glycol, ethylene glycol monophenylether, benzyl alcohol and n-propyl alcohol.